Knowledge of wellbore placement and surveying is useful for the development of subsurface oil & gas deposits, mining, and geothermal energy development. Accurate knowledge of the position of a wellbore at a measured depth, including inclination and azimuth, may be used to attain the geometric target location of, for example, an oil bearing formation of interest. Additionally, accurate relative placement of a wellbore to a geological zone or formation, or relative to one or more adjacent wellbores, may be useful for the production or hydrocarbons or geothermal energy. Additionally, knowledge of the formation properties surrounding a wellbore along its depth may be used to guide wellbore placement into desired geological features or zones. Traditionally a combination of sensors are deployed on a drill string to measure these properties.
The measurement of inclination and azimuth of the wellbore may be used in surveying operations. Inclination is the angle between the longitudinal axis of a wellbore or a drill string or other downhole tool positioned in a wellbore and the Earth's gravity vector, and azimuth is the angle between a horizontal projection of the longitudinal axis and north, whether measured by a magnetometer (magnetic north) or by a gyro (true north). In some instances, surveying operations may include estimating the distance, orientation, or both the distance and orientation of a borehole relative to other boreholes by measuring the magnetic field that is produced either passively from the adjacent wellbore's casing or drill pipe, or by measuring an actively generated magnetic field.
One method of determining the orientation and position of a downhole tool with respect to the Earth spin vector is to take a gyro survey, referred to herein as a gyrocompass, to determine a gyro toolface, inclination, and azimuth. The gyrocompass utilizes one or more gyroscopic sensors, referred to herein as gyros to detect the Earth's rotation and determine the direction to true north from the downhole tool, the reference direction for a gyro toolface and azimuth.
The determination of orientation, position, inclination, and azimuth of the downhole tool may include determining a gravity toolface or magnetic toolface by using one or more accelerometers or magnetometers respectively. Accelerometers may be used to detect the local gravity field, typically dominated by the Earth's gravity, to determine the direction to the center of the Earth. This direction may be used as the reference direction for a gravity toolface. Magnetometers may similarly be used to detect the local magnetic field, typically dominated by the Earth's magnetic field, to determine the direction to magnetic north. This direction may be used as the reference direction for a magnetic toolface.
These measurements, referred to herein as a survey, may be taken by a surveying tool positioned on a drill string. A drill string is typically made up of tubular segments joined end to end, which are built up or broken down on a drilling rig to increase or decrease respectively the overall length of the drill string during the drilling or completions process. Typically, tubular segments are joined in sections of two or three before being added to the drill string, or are removed in sets of two or three, referred to herein as a pipe stand. While the pipe stand is joined to or removed from the drill string, the drill string containing the survey or LWD tool may typically be held relatively stationary. These conditions during the joining and removal process may be ideal for making survey or logging while drilling (LWD) measurements.